Powder coating equipment having zero-sum control of conveyance and supplement air lines

ABSTRACT

Powder-coating equipment comprises two throttles ( 6, 16 ) each mounted in one of a conveyance-air line ( 8 ) and a supplemental-air line ( 18 ). The throttles&#39; flow impedance’ are adjustable in a motorized manner by providing throttle with its own adjustment motor ( 4, 14 ). The adjustment motors, such as electric stepping motors and/or servo-motors are driven by an electronic control unit ( 2 ).

TECHNICAL FIELD OF THE INVENTION

The invention relates to powder coating equipment comprising a line forconveyance air and at least one line of supplemental air, both beingconnected to an injector to pneumatically move coating powder using theconveyance air of the conveyance line and the supplemental air of the atleast one supplemental-air line.

BACKGROUND ART

As shown by the versatile state of the art, diverse research anddevelopment has followed many approaches to attain optimal operationalsetpoints using simple steps. However success so far has been elusive.The German patent 1 266 685 discloses the basic principle of an injectorpneumatically moving coating powder. The European patent document 0 412289 B discloses an electrostatic powder coating system containing adisplay for the total volumetric flow in a feed line of compressed airto the conveyance-air line and to the supplemental air line and apressure regulator in each of a conveyance-air line and asupplemental-air line. When changing the total volumetric flow byadjusting its associated pressure regulator for the purpose ofcommensurately changing the flow of powder, the entailed change in thevolumetric flow may be eliminated again by correspondingly adjusting thepressure regulator of the supplemental air line. The pressure regulatorsalso may be automatically controlled by a microcomputer instead of beingcontrolled manually. A minimum air flow is required in a powder linefrom the injector to a spray device or to a container in order to avertpowder deposits and flow pulsations in the powder line. The powder flowin the powder line should be as constant as possible, i.e., it shouldnot unduly fluctuate. On that account more supplemental air must beadded if the rate of conveyance air must be reduced to such an extentwhen a small powder flow is desired that—absent this supplementalair—the total air no longer would suffice. Beyond a partial-vacuum zoneof the injector wherein the conveyance air aspirates powder, thesupplemental air is introduced into the flow of powder/conveyance-air.In a design variation, or additionally, supplemental air also may beintroduced into the partial-vacuum zone in order to vary thepartial-vacuum generated by the conveyance air. As a result, whenreducing the volumetric flow of conveyance air, the volumetric flow ofsupplemental air will be increased, and vice-versa. A similar systemhaving a pressure regulator in each line of conveyance air and in eachline of supplemental air is known from FIG. 4 of U.S. Pat. No.3,625,404. Furthermore air-dividing valves are known from said US patentand from the German patent document 44 09 493 A which are fitted with athrottling valve in the conveyance-air line and a throttling valve inthe supplemental-air line, said valves being mechanically interlinkedand adjusted manually or using motors, whereby, as one throttle is beingopened wider, the other shall be closed further. FIG. 3 of said USpatent shows an air-divider valve of which the valve chamber and thevalve seat can be manually set relative to each other to attain asetpoint for one air flow being larger or smaller than the other.However such mechanically interlinked throttling valves incur thedrawback that the setpoint of the differential of the volumetric flowsof conveyance air and supplemental air shall be applicable only for veryspecific kinds of powder and only for a very specific configuration ofthe powder-coating system, so that, when changing to different kinds ofpowders, or when changing system components affecting flow conditions,and when changing the flow or timing at which the objects to be coatedare being moved past a spray device, said setpoint will not beautomatically variable, and new settings shall require interrupting theautomated coating procedure. Another drawback of this state of the artis that such manually implemented setpoints require considerableoperator experience to secure identical setpoints for recurringidentical coating processes. The known mechanical air dividers also canbe manufactured only with great difficulty to offer the required highaccuracy. On the other hand, using pressure regulators instead of suchair dividing valves also incurs the drawback that the volumetric airflow and the powder rate are non-linearly related to the pressures ofthe conveyance air and of the supplemental air.

SUMMARY OF THE INVENTION

This problem is solved by inventive coating equipment comprising acontainer for containing a coating material, an injector communicatedwith the container, and a conveyance-air line and at least onesupplemental air line connected to the injector. The lines supplyconveyance air and supplemental air to the injector for pneumaticallysucking the coating material from the container into the injector andmoving the sucked coating material toward an object to be coated. Aplurality of throttles each mounted in one of the conveyance-air lineand the at least one supplemental-air line are equipped with adjustmentmotors for adjusting a cross section of the throttle, and hence, an airflow of the line. The equipment further comprises an electronic controlunit that electrically and mutually adjusts the adjustment motors of allthe throttles as a function of a setpoint of a volumetric total air flowof the conveyance air and the supplemental air, and as a function of asetpoint of a material rate at which the coating material is to beapplied to the object.

This problem is solved by the invention by means of the features ofclaim 1.

In accordance with the present invention, when an electric steppingmotor is used to adjust the throttles, feedback of actual values to thecontrol unit no longer is required for purposes of control or regulationbecause the control unit intrinsically always knows how many steps thestepping motor has carried out in either direction of rotation. Thecontrol unit is wholly electronic and preferably comprises one orseveral microprocessors.

The control unit is wholly electronic and preferably comprises one orseveral microprocessors.

The setpoints for the total volumetric flow, for the required rate ofpowder and other values may be fed to the control unit. Moreover thecontrol unit can be controlled or regulated automatically by anoverriding control center or main computer as a function of objects tobe coated, for instance when there is a change in the kind of powder, inthe desired coating thickness and/or timing or the conveyance rate atwhich the objects being coated move past a spray device.

BRIEF DESCRIPTION OF THE DRAWING

The invention is elucidated below by means of a preferred andillustrative embodiment.

The single FIG. 1 schematically shows powder-coating equipment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The power-coating equipment of the invention shown in FIG. 1 comprisesan electronic control unit 2 which controls an electric stepping motor14 electrically adjusting a variable flow throttle 16 in a supplementalair line 18 through electric lines 10 or 20 as a function of a setpointpowder rate (quantity of powder per unit time) and as a function of asetpoint total volumetric flow of air (total air per unit time)consisting of conveyance air of the conveyance-air line 8 andsupplemental air of the supplemental-air line 18. There are alsoprovided a key 22 to raise the setpoint of powder rate, a key 23 toreduce the setpoint powder rate, a key 24 to raise the setpointvolumetric air flow and a key 25 to reduce the setpoint volumetric airflow are used. Each key is driven, e.g., manually. In an embodimentvariation, the electronic control unit 2 also may contain one or morecomputer programs to automatically adjust the setpoints of the powderrate and the total volumetric air flow as a function of the objects tobe coated, further of their speed or their timing, of the desiredcoating thickness, the kind of powder to be used (granularity, plastic,ceramics) and/or other criteria. Such computer program(s) may be housednot in the control unit 2 but instead in an overriding control center 30exchanging data through a data bus 32 with the control unit 2.

The upstream ends of the conveyance-air line 8 and the supplemental-airline 18 are connected through a pressure regulator 34 to a common sourceof compressed air 36.

The downstream ends of the conveyance-air line 8 and thesupplemental-air line 18 are connected to an injector 38. Theconveyance-air flow of the conveyance-air line 8 moves from an injectornozzle 40 through a partial-vacuum zone 42—wherein said flow generatesthe partial vacuum and thus aspirates coating powder 46 out of a powdercontainer 44—and then into collecting duct 48 and from there, togetherwith the aspirated powder, through a powder line 50 into a furthercontainer, or, as shown in FIG. 1, to a spray device 52. Preferably thespray device 52 is designed to electrostatically charge the powder. Itmay be in the form of a manual or automatic spray gun or a rotationalatomizer or the like. Said spray device sprays the pneumaticallyconveyed powder 46 for the objects 54 to be coated, said objects beingautomatically moved by a conveyor system 56 past the spray device at itsspraying side.

A powder suction aperture 58 of the injector 38 issues into the partialvacuum zone 42. Instead of being configured underneath the injector 38,the container 44 also may be mounted above it.

The downstream end of the supplemental air line 18 is connected to asupplemental-air aperture 60 of the injector 38 from where saidsupplemental air flows into the powder/conveyance-air flow of thecollecting duct 48.

To change the rate of conveyed powder, or to keep it constant when thekind of powder changes, the volumetric conveyance air of the conveyanceair line 8 is correspondingly adjusted by driving the adjustment motor 4at the variable throttle 6 by means of the control unit 2. In order tocompensate the total air content which is changed thereby in theair/powder flow, the volumetric supplemental air flow of thesupplemental air line 18 is also correspondingly adjusted by adjustingits throttle 16 using the adjustment motor 14.

Several lines instead of the single line of supplemental air 18 also maybe used and may issue into the collecting duct 48 or upstream ordownstream from it into the flow path of the powder and conveyance air.

Other motors than the electric stepping motors 4 and 14 acting asadjustment motors can be used, for instance servo-motors. In all casesthe mutual control or regulation of the throttles 6 and 16 by theirmotors is carried out in purely electrical manner by means of theelectronic control unit 2. Electrical stepping motors offer theadvantage over hydraulic motors that the control unit 2 “knows” at alltimes the adjustment of the pertinent throttle 6 or 16 because itautomatically knows how many steps were taken by the stepping motor ineither direction of rotation.

The throttles 6 and 16 may be adjustable diaphragms or adjustable valvesor cocks.

A display 60 shows the setpoint values and the actual values of thevolumetric flow of conveyance air, of the flow of supplemental air, ofthe total volumetric flow of conveyance air and supplemental air, and ofthe rate of powder.

The rate of powder is approximately proportional; to the flow (quantityper unit time) of conveyance air in the conveyance-air line 8.Accordingly the conveyance air need only be adjusted by means of thekeys 22 and 23 in order to set a desired powder rate. The control unit 2then automatically sets the rate of supplemental air by means of theadjustment motor 14 and the throttle 16 in such manner that in spite ofthe change in rate of conveyance air, the rate of the total volumetricair flow (total air rate) remains at the setpoint value in effect.

At constant air pressure at the pressure regulator 34, the rate ofconveyance and that of the supplemental air will only changeproportionately to a change in the flow cross-sections of theirthrottles 6 and 16 if the downstream flow impedance is very small.However, as regards equipment of the present design comprising aninjector and a powder line 50 however, the flow impedance is so largethat the rates of conveyance air and of supplemental air do not changelinearly in relation to changes in the flow cross-sections of thethrottles 6 and 16. In a preferred embodiment of the invention, thenon-linear function of at least one, or several flow impedance(s)(different injector 38 and/or powder lines 50) is stored in the form ofplots in the control unit 2, and this control unit non-linearly drivesthe throttles 6 and 16, by means of the adjustment motors 4 and 14, as afunction of setpoint values, for instance at the keys 22, 23, 24 and 25,in such manner that a change in setpoint value entails a linear changeof the rate of conveyance air and/or that of the supplemental air.

In a preferred implementation of the invention, the spray device 50 is amanual spray gun on which are mounted the finger-actuated keys 22 and23, preferably also the keys 24 and 25.

What is claimed is:
 1. Coating equipment, comprising: a container forcontaining a coating material; an injector communicated with saidcontainer; a conveyance-air line and at least one supplemental air lineconnected to said injector and supplying conveyance air and supplementalair, respectively, to said injector for pneumatically sucking thecoating material from said container into said injector and moving thesucked coating material toward an object to be coated; a plurality ofthrottles each mounted in one of the conveyance-air line and the atleast one supplemental-air line and equipped with an adjustment motorfor adjusting a cross section of said throttle, and hence, an air flowof said line; and an electronic control unit that electrically andmutually adjusts the adjustment motors of all said throttles as afunction of a setpoint of a volumetric total air flow of the conveyanceair and the supplemental air, and as a function of a setpoint of amaterial rate at which the coating material is to be applied to saidobject.
 2. The equipment of claim 1, wherein the electronic control unithas data on adjustment position and direction of at least one of saidadjustment motors, and a current state of the cross section of thethrottle adjusted by said adjustment motor is always determinable by theelectronic control unit.
 3. The equipment of claim 1, wherein thecoating material is a powder.
 4. The equipment of claim 3, wherein theelectronic control unit has data on adjustment position and direction ofat least one of said adjustment motors, and a current state of the crosssection of the throttle adjusted by said adjustment motor is alwaysdeterminable by the electronic control unit.
 5. Coating equipment asclaimed in claim 1, wherein of conveyed air by means of said controlunit (2) driving the particular adjustment motor (4, 14) in non-linearmanner; the air flow of at least one of said lines changes as anon-liner function of the cross section of the throttle installed insaid line; said non-liner function is stored in the electronic controlunit; and using said stored non-linear function, the electronic controlunit non-linearly drives the adjustment motor of the throttle installedin said line so that the air flow in said line changes as a linearfunction of said setpoints.
 6. The equipment of claim 2, wherein theelectronic control unit stores a plurality of said non-linear functionscorresponding to various flow impedances of various configurations of aflow path which is downstream of the throttle installed in said line andincludes said injector.
 7. The equipment of claim 5, wherein theelectronic control unit has data on adjustment position and direction ofat least one of said adjustment motors, and a current state of the crosssection of the throttle adjusted be said adjustment motor is alwaysdeterminable by the electronic control unit.
 8. The equipment of claim7, wherein said non-linear function is stored in the electronic controlunit as a plot.
 9. The equipment of claim 5, wherein said non-linerfunction is stored in the electronic control unit as a plot.
 10. Theequipment of claim 5, wherein the coating material is a powder.
 11. Theequipment of claim 10, wherein the electronic control unit has data onadjustment position and direction of at least one of said adjustmentmotors, and a current state of the cross section of the throttleadjusted by said adjustment motor is always determinable by theelectronic control unit.
 12. The equipment of claim 11, wherein saidnon-liner function is stored in the electronic control unit as a plot.13. Powder-coating equipment, comprising: a container for containing acoating powder; an injector communicated with said container; aconveyance-air line and at least one supplemental air line connected tosaid injector and supplying conveyance air and supplemental air,respectively, to said injector for pneumatically sucking the coatingpowder from said container into said injector and moving the suckedcoating powder toward an object to be coated; a plurality of throttleseach mounted in one of the conveyance-air line and the at least onesupplemental-air line and equipped with an adjustment motor foradjusting a flow impedance of said line; and an electronic control unitthat electrically and mutually adjusts the adjustment motors of all saidthrottles as a function of a setpoint of a volumetric total air flow ofthe conveyance air and the supplemental air, and as a function of asetpoint of a powder rate at which the coating powder is to be appliedto said object; wherein the adjustment motors are electrical steppingmotors.
 14. Powder coating equipment as claimed in claim 13, wherein ofconveyed air by means of said control unit (2) driving the particularadjustment motor (4, 14) in non-linear manner; an air flow of at leastone of said lines changes as a non-liner function of a cross section ofthe throttle installed in said line; said non-liner function is storedin the electronic control unit; and using said stored non-linearfunction, the electronic control unit non-linearly drives the adjustmentmotor of the throttle installed in said line so that the air flow insaid line changes as a linear function of said setpoints.
 15. Theequipment of claim 14, wherein the electronic control unit stores aplurality of said non-linear functions corresponding to various flowimpedances of various configurations of a flow path which is downstreamof the throttle installed in said line and includes said injector. 16.The equipment of claim 14, wherein said non-linear function is stored inthe electronic control unit as a plot.
 17. Powder-coating equipment,comprising: a container for containing a coating powder; an injectorcommunicated with said container; a conveyance-air line and at least onesupplemental air line connected to said injector and supplyingconveyance air and supplemental air, respectively, to said injector forpneumatically sucking the coating powder from said container into saidinjector and moving the sucked coating powder toward an object to becoated; a plurality of throttles each mounted in one of theconveyance-air line and the at least one supplemental-air line andequipped with an adjustment motor for adjusting a flow impedance of saidline; and an electronic control unit that electrically and mutuallyadjusts the adjustment motors of all said throttles as a function of asetpoint of a volumetric total air flow of the conveyance air and thesupplemental air, and as a function of a setpoint of a powder rate atwhich the coating powder is to be applied to said object; wherein theadjustment motors are servo-motors.
 18. Powder coating equipment asclaimed in claim 17, wherein an air flow of at least one of said lineschanges as a non-liner function of a cross section of the throttleinstalled in said line; said non-liner function is stored in theelectronic control unit; and using said stored non-linear function, theelectronic control unit non-linearly drives the adjustment motor of thethrottle installed in said line so that the air flow in said linechanges as a linear function of said setpoints.
 19. The equipment ofclaim 18, wherein the electronic control unit stores a plurality of saidnon-linear functions corresponding to various flow impedances of variousconfigurations of a flow path which is downstream of the throttleinstalled in said line and includes said injector.
 20. The equipment ofclaim 18, wherein said non-linear function is stored in the electroniccontrol unit as a plot.